This invention relates to a multiplex communication system and is particularly, but not exclusively, intended for use in motor vehicles.
Conventional practice in motor vehicles has been to provide each powered device (light, horn, windscreen wiper motor, etc.) with its own power lead and associated driver's control switch, and to provide a number of warning and indicating instruments (fuel gauge, tachometer, oil pressure warning, etc.) each connected to an appropriate sensor by separate wiring. This gives rise to wiring looms of considerable complexity and cost. It is also necessary to fabricate and stock different wiring harnesses for each model of vehicle.
There have hitherto been a considerable number of proposals to overcome these problems by using a common channel interconnecting all electrically powered devices and monitoring devices with a central control station, information being passed along the channel by multiplexing techniques. None of these proposals has yet been put into practice in volume vehicle production, principally for reasons of cost, and/or complexity. The factors which can be identified as necessary for a commercially viable system are:
(a) the system must be mechanically simple and robust;
(b) the number of different components required must be kept to a minimum;
(c) the system must be sufficiently fast to maintain information such as road and engine speed sufficiently up to date in real time for the purposes of the driver;
(d) it must be possible to control at least 50 functions and to receive information from a similar number of sensors; and
(e) there must be signal security which prevents spurious signals caused by interference effecting erroneous operation of controlled devices.
Of the systems previously proposed, some have been too slow or have too small a channel capacity to be suitable for use in vehicles, while others have achieved the required speed and channel capacity by using long trains of pulses at high repetition rates which requires the use of high frequency components with attendant expense. Other systems are unsuitable because they require a number of signal-carrying conductors, which increases cost and the risk of incorrect connection.